1. Field of the Invention
This disclosure is related to the field of polymer interlayers for multiple layer glass panels and multiple layer glass panels having at least one embossed polymer interlayer sheet. Specifically, this disclosure is related to the field of embossed polymer interlayer sheets of multiple layer glass panels and methods of embossing the polymer interlayer sheets.
2. Description of Related Art
Generally, multiple layer glass panels are comprised of two sheets of glass, or other applicable substrates, with a polymer interlayer sheet or sheets sandwiched there between. The following offers a simplified description of the manner in which multiple layer glass panels are generally produced. First, at least one polymer interlayer sheet is placed between two substrates to create an assembly. It is not uncommon for multiple polymer interlayer sheets to be placed within the two substrates creating a multiple layer glass panel with multiple polymer interlayers. Then, air is removed from the assembly by an applicable process or method known to one of skill in the art; e.g., through nip rollers, vacuum bag or another de-airing mechanism. Following the removal of the air from the assembly, the constituent parts of the assembly are preliminarily press-bonded together by a method known to one of ordinary skill in the art. In a last step, in order to form a final unitary structure, this preliminary bonding is rendered more permanent by a lamination process known to one of ordinary skill in the art such as, but not limited to, autoclaving. Amongst other applications, the resultant laminate glass panels from this process are utilized in architectural windows and in the windows of motor vehicles and airplanes.
Generally, there are a number of possible problems encountered in the art of manufacturing multiple layer glass panels. Two (2) common problems are: de-gassing and optical quality.
De-gassing (also referred to as de-airing) is the removal of the presence of gas or air in a multiple layer glass panel. Gas trapped in a multiple layer glass panel can have a negative or degenerative effect on the optical clarity and adhesion of the panel. During the manufacturing process of laminated multiple layer glass panel constructs, gases can become trapped in the interstitial spaces between the substrates and the one or more polymer interlayers. Generally, this trapped air is removed in the glazing or panel manufacturing process by vacuum de-airing the construct, nipping the assembly between a pair of rollers (also referred to as nip roll de-airing) or by some other method known to those of skill in the art. However, these technologies are not always effective in removing all of the air trapped in the interstitial spaces between the substrates, especially when the polymer interlayer sheet has a smooth surface.
Lamination of a multiple layer glass panel is a multi-step process in which the polymer interlayer sheet(s) and rigid substrates are converted to a combined final form (the multiple layer glass panel or glazing) having desirable performance and optical clarity characteristics. Nip roll de-airing is one technique that is used to evacuate air from the rigid substrate/polymer interlayer/rigid substrate construction prior to the final step in the lamination process of autoclaving. It frequently can be employed to improve autoclave yields in commercial operations. In one embodiment, samples are moved along a conveyor in an oven or several ovens and then forced into and through a pair of rubber rollers with a set gap. The gap is typically set slightly smaller than the thickness of the sample (i.e., the substrate/polymer/substrate construction). The nip rollers typically can range in hardness, for example, from 35 to 75 Durometer Shore A. The conveyor transporting the sample through the oven prior to where the sample is “nipped” heats up the glass. Glass temperatures of the sample can be from 30° C. to over 100° C., as desired. Lower temperatures are often referred to as a “cold” nip roll process. As used herein, when referring to a cold nip roll process, the glass temperature ranges from about 45° C. to about 60° C. (as measured by a contact thermometer or similar device attached to or aimed at the glass, such as with an infrared type thermometer). Once cooled to room temperature, the nipped sample can be evaluated for de-airing quality or uniformity. The nipped sample can then be placed in an autoclave for final finishing. After finishing, the final or finished laminate can be evaluated for mottle and other optical defects.
Generally, the presence of a gas in the interstitial spaces of a multiple layer glass panel takes the form of bubbles in the polymer interlayer sheet(s) or pockets of gas between the polymer interlayer sheet(s) and the substrates. These bubbles and gaseous pockets are undesirable and problematic where the end-product multiple layer glass panel will be used in an application where optical quality is important. Thus, the creation of a solid-phase interlayer essentially free of any gaseous pockets or bubbles is paramount in the multiple layer glass panel manufacturing process.
Not only is it important to create a multiple layer glass panel free of gaseous pockets and bubbles immediately after manufacturing, but it is also important that the glass panel remain free of gaseous pockets and bubbles over its lifetime. It is not an uncommon defect in the art of multiple layer glass panels for dissolved gases to appear (e.g., for bubbles to form) in the panel over time, especially at elevated temperatures and under certain weather conditions and sunlight exposure. Thus, it is also important that, in addition to leaving the laminate production line free from any bubbles or gaseous cavities, that the multiple layer glass panel remain gas-free for a substantial period of time under end-use conditions to fulfill its commercial role.
For the successful production of high quality automotive or architectural multiple layer glass panels, also known as safety glazings, there needs to be an assurance of high yields when combining the initial, individual, polymer sheet(s) with the preferred glass type/part through the entire glass panel manufacturing process. A key part of that process is the autoclave cycle where the multiple layer glass panel structure enters into a pre-final or “pre-press” stage and emerges as a completed entity and saleable part for installation into a vehicle or building. Ensuring the highest autoclave yield possible for the glass panel manufacturer is essential for highest profitability and success because production of multiple layer glass panels is a time-consuming batch process where significant process bottlenecking and highest losses can occur because it is difficult to recycle parts once they have been laminated.
In order to facilitate the de-airing process and to provide acceptable optical quality, particularly in the glass panel, it has become common in the art of multiple layer glass panel manufacturing to emboss one or both sides of the polymer interlayer(s), thereby creating minute raised and depressed portions on the surface of the polymer interlayer. Embossment of the polymer interlayer has been shown to be effective in enhancing the de-airing process, and it is also effective in reducing the occurrence of blocking.
While certain embossing methods and techniques in the manufacture of multiple layer glass panels are known, there are several problems with the different embossing processes previously utilized in the art (referred to herein as “Conventional Processes”). In some Conventional Processes for making polymer interlayer sheets, the polymer sheet was cooled from a polymer sheet melt to form a polymer interlayer sheet, and then the surface of the polymer interlayer sheet was reheated, before the embossing step. Practically, in some methods, this necessitated that the polymer interlayer be fed through multiple sets of rollers in additional production steps before it could be embossed. FIGS. 1 and 2 depict two different Conventional Processes each which utilize multiple cooling, reheating and embossing steps. These additional production steps could significantly add to the costs, energy intake and the overall space required for multiple layer glass panel production.
Often, if both sides of a polymer were embossed in the Conventional Processes, the embossing was generally performed in separate successive steps with separate sets of embossing rollers by running the polymer interlayer sheet between two sets of embossing rollers. Thus, embossing in some Conventional Processes was performed in multiple separate successive stages with different sets of rollers, with each side of the polymer interlayer sheet being embossed in one of the successive stages. FIG. 2 provides a diagram of such a multi-step embossing process.
This multi-stage embossing process is generally required in some Conventional Processes because of the necessity of cooling the polymer interlayer sheet from a melt prior to embossing. As noted previously, in some Conventional Processes, the polymer interlayer sheet is not embossed directly after it leaves the extrusion die while it is still a melt because the molten polymer will stick to the embossing rolls causing a mess and degrading the integrity of the polymer interlayer sheet, rendering it unusable. Accordingly, the polymer interlayer sheet is cooled prior to embossing. However, a completely cooled polymer interlayer sheet is difficult, if not impossible, to emboss, therefore, in some Conventional Processes, after the polymer melt is cooled to a polymer interlayer sheet, the surface of the interlayer sheet must be reheated with the embossing roller (or by some other technique) at the time of embossing. The surface roughness or surface pattern of the embossed polymer interlayer sheet made by this process often has lower retention of the pattern than surface roughness or surface pattern of embossed polymer interlayer sheets of some other processes.
In some Conventional Processes using two embossing steps, the heated embossing roller is combined with a non-embossing roller, such as a rubber roller, which offers greater and more consistent pressure (higher contact force) to the embossing roller system than can be achieved if two metal (e.g., steel) embossing rollers are utilized simultaneously. Thus, if both sides of the polymer interlayer sheet are to be embossed in the Conventional Processes, usually at least two sets of rollers (each set being comprised of an embossing roller and a rubber roller) are utilized.
FIG. 3 depicts another Conventional Process for embossing a polymer interlayer sheet. As shown in FIG. 3, the polymer interlayer sheet is embossed in a step right after leaving the extruder die. The polymer interlayer sheet is embossed at an elevated temperature (that is, it is embossed while it is still a melt). No cooling step is required or utilized to lower the temperature between the steps of extrusion from the extrusion die and embossing. Rather, the polymer melt sheet (as opposed to the cooled and set polymer interlayer sheet) is embossed in a single embossing stage in which the polymer melt sheet is fed from the extrusion die into a single set of two embossing rollers (which in some embodiments are made of steel) directly out of the extrusion die, and both sides of the polymer melt sheet are simultaneously embossed. One side of the polymer melt sheet is embossed by one of the embossing rollers and the other side of the polymer melt sheet is embossed by the other embossing roller.
In some Conventional Processes, the embossed polymer interlayer sheet would perform adequately in the de-airing processes, but the optical clarity or optical properties of the final multiple layer glass panel was not acceptable to laminators due to reduced optical clarity or mottle. In other Conventional Processes, the optical clarity or optical properties of the final multiple layer glass panel was acceptable to laminators, but the embossed polymer interlayer sheet would not perform adequately in the de-airing processes, and de-air uniformity was unacceptable. Finally, in some Conventional Processes, neither the de-airing performance nor the optical quality (or mottle) was adequate. Mottle refers to an objectionable visual defect that manifests itself as graininess or texture in a laminated multiple layer polymer interlayer. Mottle and de-air uniformity will be further described below.
Multiple layer glass panels or laminated safety glazings need to have high optical quality and be free of defects as they are used in many demanding visual applications. It has been found that the quality of the pre-press laminate (the rigid substrate/polymer/rigid substrate construction or the glass/polymer sheet(s)/glass sandwich construction) after initial de-airing, such as cold nip roll de-airing, prior to final lamination is an excellent predictor of how well the panels will behave in the autoclave (that is, how good the final laminate quality will be) and therefore how good or bad the autoclave yield will be, therefore, the pre-press laminates should have certain qualities such as minimum entrapped air, a uniform de-airing appearance and minimization of air pockets or surface texture prior to autoclave finishing steps.
Summarized, the previous embossed polymer interlayer sheets, when laminated in a multiple layer glass panel, did not always provide the combination or balance of desired optical clarity (such as mottle) and optimum de-airing properties (such as de-air uniformity in the cold nip roll process). Stated differently, previous embossed polymer interlayer sheets, as well as some non-embossed polymer interlayer sheets, did not always provide the desired balance of properties, including at least good or acceptable de-airing and good or acceptable optical clarity, while not adversely affecting other properties of either the embossed polymer interlayer sheet or the multiple layer glass panel. This embossed polymer sheets of the invention specifically provide an embossed polymer sheet that has good de-air uniformity in the cold nip roll process and low, commercially acceptable mottle.